Hereditary spastic paraplegias (HSP) are genetically heterogeneous Mendelian disorders characterized by weakness, spasticity and loss of vibratory sense in the lower limbs (Harding et al. 1983; Tallaksen et al. 2001; Fink et al, 2006; Depienne et al, 2007; Stevanin et al, 2008). They reveal themselves clinically through difficulties in walking possibly evolving into total paralysis of both legs. The physiopathology of this set of diseases is, to date, relatively undocumented; however, anatomopathological data make it possible to conclude that the attack in pure forms of the disease is limited to the pyramidal tracts responsible for voluntary motricity in the spinal cord. The incidence of HSPs, which remains difficult to estimate because of rare epidemiological studies and the considerable clinical variability, varies from 0.9:100000 in Denmark, 3 to 9.6:100000 in certain regions of Spain (Polo et al., 1991) 5:100000 in South-Tunisia (Boukrhis et al, 2009) or 14:100000 in Norway (Skre, 1974) (approximately 3:100000 in France). Various clinical and genetic forms of HSP exist. The so-called “pure” HSPs, which correspond to isolated spasticity of the lower limbs, are clinically distinguished from the “complex” HSPs, for which the spasticity of the legs is associated with other clinical signs of neurological or non-neurological type.
Although forms of HSP have been recognized for over a century, new phenotypes are regularly described, demonstrating wide clinical heterogeneity. Genetically, autosomal dominant (AD), autosomal recessive (AR) and X-linked inheritance are observed and almost 34 genetic loci have been identified, but only 17 genes have been cloned (Depienne et al, 2007; Stevanin et al, 2008a). According to the putative roles of these genes, mitochondrial function, protein folding, abnormal development, cholesterol/neurosteroid metabolism and axonal transport have been implicated in the dying back of pyramidal tract axons in these disorders (Stevanin et al, 2008a).
The most common forms of AD-HSP, accounting for about 40-50% of cases (Depienne et al, 2007; Stevanin et al, 2008a), are caused by mutations in the SPG4 and SPG3A genes that encode for spartin and atlastin, respectively (Hazan et al. 1990, Zhao et al. 2001 and international patent application WO 01/18198). In AR-HSP, which is less common and more varied in clinical presentation, greater genetic heterogeneity is expected but SPG11 (patent application No 06 291 433.8) was found to be frequently mutated, accounting for ˜21% of all ARHSP, but up to 59% of ARHSP with thin corpus callosum and mental impairment (Stevanin et al, 2007 and 2008b). The five other AR-HSP genes cloned so far (details are available on the “Online Mendelien Inheritance in Man” database at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM), encoding for CYP7B1 (SPG5, MIM#270800, Tsaousidou et al. 2008), paraplegin (SPG7, MIM#607259, Casari et al. 1998), spartin (SPG20; MIM#275900, Patel et al. 2002) and maspardin (SPG21, MIM 248900, Simpson et al. 2003) as well as the gene responsible for the related spastic ataxia of Charlevoix Saguenay (ARSACS, MIM#270550, Engert et al. 2000) probably represent less than 10% of all cases (Depienne et al, 2007; Stevanin et al, 2008a).
In a clinical point of view, a very common form of AR-HSP associates spastic paraplegia, mental or cognitive deficit and thin corpus callosum (Martinez et al 1999, Shibasaki et al. 2000, Casali et al. 2004, Winner et al. 2004 and 2005, Lossos et al. 2006, Stevanin et al. 2006, Franca et al, 2007, Boukhris et al 2008a). The majority of the families appear to be linked to the SPG11 gene (Stevanin et al, 2007, Stevanin et al, 2008b). In the patent application No 06 291 433.8 related to this SPG11 gene, the inventors claimed to have identified a gene responsible for a frequent form of Autosomal Recessive Hereditary Spastic Paraplegia (AR-HSP). They have indeed demonstrated that the disease is caused by mutations in the KIAA1840 gene (also known as FLJ21439), affecting the spatacsin protein expression, which was further confirmed independently by other groups (for review, see Stevanin et al, 2008a). The typical clinical features of this disease consist of early-onset spastic paraplegia (usually <20 years), urinary bladder dysfunction, deep sensory deficits in the legs and cognitive impairment that progress insidiously to severe functional disability over a period of 10-20 years (Winner et al, 2005; Franca et al, 2007, Boukhris et al 2008a). Some patients also develop arm involvement, dysarthria, contractures and muscle atrophy. Auxiliary studies frequently identify a thin corpus callosum (TCC) with white matter lesions and variable cerebral cortical atrophy on magnetic resonance imaging (MRI), variable cortical and thalamic glucose hypometabolism on positron emission tomography and predominantly axonal motor or sensorimotor peripheral neuropathy on nerve conduction studies (Winner et al. 2004).
Other loci have been found associated with this phenotype however: SPG15 (Hughes et al 2000), SPG21 (Simpson et al, 2003), HSP with epilespy (Al-Yahyaee, 2006) and occasionally SPG7 (Coutinho et al, 1999) or SPG4 (Orlacchio et al, 2004). More particularly, SPG15 was thought to be a rare form of spastic paraplegia associated with pigmentary maculopathy, also known as the Kjellin syndrome (MIM 270700, www.ncbi.nlm.nih.gov/omim/), when it was mapped in two Irish families to a region of 16 Mb on chromosome 14q (Hughes et al, 2000). Recently, the inventors identified 7 families linked to SPG15 locus (Elleuch et al, 2007; Boukhris et al, 2008a and 2008b); Muglia et al, submitted), reduced its size to 5.3 Mb between markers D14S981 and rs8688 (Elleuch et al, 2007) then between markers VNTR25TG (identified in the human genome sequence by the inventors, primers in the table 4) and D14S1029 (FIG. 1) and estimated its frequency to 15% of ARHSP (Elleuch et al, 2007). In addition, the inventors showed that the clinical features varied among patients and families, but cognitive impairment with distal amyotrophy were frequently associated. Peripheral neuropathy, thin corpus callosum, maculopathy and cerebellar ataxia were also observed in some but not all patients (Elleuch et al, 2007, Boukhris et al, 2008a and 2008b). Therefore, SPG15 is expected to account for a significant proportion of the very common sub-form of ARHSP associating mental or cognitive deficits and thin corpus callosum, as SPG11, but could also account for other forms of ARHSP since all features of this clinical entity can be absent in certain families.